Ceramic compositions based on alumina (Al.sub.2 O.sub.3) have been in use for many years. Their applications in, e.g. the cutting tool industry have been recognized and utilized extensively. However, ceramics such as Al.sub.2 O.sub.3 +ZrO.sub.2 and Al.sub.2 O.sub.3 +TiC+ZrO.sub.2 have been criticized as being brittle or difficult to fabricate into complex shapes. Additional amounts of ZrO.sub.2 have not been found to improve mechanical shock resistance and additional TiC has not significantly improved wear resistance.
The search for improved ceramic compositions has produced compositions known as SIALON, an acronym for silicon aluminum oxynitride. This composition in its most advantageous application consists primarily of beta-SIALON. However, this compound is one of the more expensive elements of ceramic composition and thus contributes to increased fabrication costs.
This difficulty has been recognized and other starting materials have been proposed which result through processing in a sialon composition from less expensive starting materials. See for example, U.S. Pat. No. 4,184,884.
Others have sought to improve the composition of ceramics available for cutting tool applications and the like through expensive fabrication techniques such as for example, hot isostatic pressing. However, this also results in increased fabrication costs even though the starting materials may be relatively abundant.
It has been recognized that the Al.sub.2 O.sub.3 component of sialon can be increased, but uncertainty exists as to how much Al.sub.2 O.sub.3 can be accomodated in this matrix. See for example U.S. Pat. No. 4,071,371 to Milberg et al.
Kamigaito et al. has disclosed a composition of Si.sub.3 N.sub.4 and Al.sub.2 O.sub.3 which also include AlN in widely varying amounts, U.S. Pat. No. 3,903,230.